In automobile engines, in order to prevent the escape of fuel vapor from a fuel tank into the atmosphere, the vapor is for example adsorbed by a canister of active carbon, and the adsorbed fuel is purged in the intake passage of the engine under predetermined running conditions. The fuel is removed from the canister and led into the intake passage by making use of the pressure difference between the atmosphere and the intake passage. For this purpose, the canister and intake passage are connected by a purge line, a purge valve which operates under predetermined running conditions being interposed in this line. Purge gas flowing into the intake passage from the purge line is led into the engine cylinder together with fuel injected by an injector provided in the intake passage, and burnt.
In many automobile engines which employ a three-way catalyst to process the engine exhaust gas, the air-fuel ratio (AFR) of the fuel mixture provided to the engine cylinder is feedback-controlled to stay in a certain region of a theoretically defined value. For example, a basic fuel amount is computed based on the intake volume of the engine, and this basic fuel injection amount is corrected based on the actual AFR detected by an oxygen sensor provided in an exhaust passage of the engine.
When the aforesaid purge gas is introduced, the fuel amount supplied to the engine cylinder increases by an amount corresponding to the purged fuel, and naturally the AFR becomes richer. In an engine with the above AFR feedback control system, the injection amount from the injector is corrected so that it is decreased.
If for example the detected AFR changes from lean to rich with reference to the theoretical value, an updating amount P is subtracted in one step from the feedback correction coefficient .alpha., and an updating amount I are then subtracted integrally until the AFR next changes back to lean. This is so-called "PI control". The correction coefficient .alpha. can therefore be varied only in fixed proportions corresponding to a fixed feedback gain, and if the AFR is varying rapidly, a certain response time is required until the AFR can be made to converge to a target value.
If now the accelerator pedal is depressed so that the vehicle accelerates while fuel is being purged from the canister, the intake amount immediately increases, but the flowrate of purge gas does not vary so much. The fuel supplied to the engine is therefore insufficient due to the fact that the purge gas amount has decreased relative to the intake amount, so this deficiency is compensated by feedback control which increases the fuel amount injected by the injector.
Just before acceleration, however, the feedback control correction coefficient .alpha. had shifted from a center value of 1.0 to lean (e.g. 0.8) so as to make the AFR converge to the target value. A relatively long time is therefore required for the coefficient to change back to rich which is necessary to increase the fuel amount so that the vehicle can accelerate, and during this time the engine does not respond properly. To deal with this problem, in Tokkai Hei 2 - 19631 published by the Japanese Patent Office, a method is proposed whereby the basic fuel amount is first decreased by a predetermined value while purge gas is being led into the cylinder, the AFR is modified to the target value (theoretical AFR) by feedback control, and the correction coefficient .alpha. is maintained close to the center value of 1.0 even during purge. For this purpose, the AFR correction coefficient .alpha. before stating purge is compared with the AFR correction coefficient .alpha. after purge which has fallen to a stable level below the predetermined value. By first subtracting a fuel correction amount corresponding to this difference from the basic fuel amount, the correction coefficient .alpha. during purge is thereby forcibly maintained in the region of 1.0 which is the center value. When the accelerator pedal is depressed, therefore, the feedback control correction coefficient .alpha. shifts to rich from the region of 1.0. Compared to the case when it shifts to rich from lean, the fuel amount increases more rapidly, hence the engine acceleration response is improved.
If the control gain of the correction coefficient .alpha. is increased, the response is improved when there are transient and other fluctuations of the AFR, but the correction width of the AFR is too large in the steady state. This causes the AFR to "hunt" around the target value which destabilizes the control. The control gain of the correction coefficient is therefore set to an appropriate value from the viewpoint of both response and stability.
In the final analysis, however, it is the fuel injection amount supplied by the injector which is corrected by the correction coefficient. More specifically, this correction is a predetermined ratio having a certain correction width which is applied to the fuel injection amount at one particular point in time, and it takes no account of fuel changes due to purge gas.
In actual fact, the AFR varies with the concentration and flowrate of purge gas, and if the AFR is corrected by feedback control, the width of the AFR which can be corrected by the correction coefficient is smaller the larger the effect of the purge gas. As a result, even if the correction coefficient .alpha. is maintained at a value near its center value during purge, it takes longer to make the AFR converge to a target value when the running conditions of the vehicle change and fuel fluctuations occur due to purge than when purge is not carried out. This leads to a worsening of exhaust gas composition during the period in question, and feedback control response during purge deteriorates.